84
THE ESSEX NATURALIST
heather spaces. In the middle ages much of it was Royal Forest,
set apart for the King's pleasure of hunting and Epping Forest
is all that is left of this.
The map shows the Watsonian (dashed line) and administra-
tive boundaries of the county, as well as the 10 km squares. The
county is bounded in the south by the River Thames, in the west
from Middlesex by the River Lea, for a certain distance from Hert-
fordshire by the River Stort, an administrative boundary from
Cambridgeshire and is separated from Suffolk by the River Stour.
The division into two vice counties follows H. C. Watson
(1859). North Essex (v.c. 19) is separated from South Essex
(v.c. 18) by the high road (A121 and All) from Waltham Abbey
to Epping and to Chelmsford (A122) and thence by the River
Blackwater to the coast. No subsequent administrative altera-
tions to county boundaries are recognised in botanical records.
The chief alterations of this kind, apart from the recent changes
in connection with the formation of Greater London, are found in
the North-West of the coutnty, but two or three very small parts
on the north bank of the River Thames near North Woolwich
botanically speaking belong to Kent.
Essex has no land over 400 ft (120m) and the geological map
shows that it is a district of Tertiary clays and sands (Geological
Museum). In general, v.c. 18 is London clay country and v.c.
19 Boulder clay. The London clay is bluish, weathering brown
and reaches its maximum thickness at Brentwood, 430 ft (131m).
Its lower part is sandy with flint pebbles. The Bagshot Sands,
marine in origin, are fine light buff but sometimes crimson sands,
and are scattered across the south of the county, for example at
High Beach, Kelvedon Hatch and at Rayleigh. They are
deposited on the clay and probably have a maximum thickness
of 120 ft (36m). Underlying the Tertiary Clay is the chalk which
comes to the surface in the south-west at Purfleet and Grays,
and in the north-west around Saffron Walden. The chalky
boulder clay is bluish gray, mixed with chalk and flints, which
must have been brought down from the north by the glaciers.
At Walton-on-the-Naze, at Wrabness, and elsewhere in the north-
east there are small amounts of the Pleistocene ferruginous sand,
the Red Crag.
The Meteorological Office states that the county is the driest.
in the British Isles. The average annual rainfall varies between
26.5 in (66 cm) at High Beach, Loughton to 20 in (50 cm) on the
coast. Hatfield Broad Oak and North Weald have 25 in (63 cm),
Ingatestone 24 in (61 cm), Lexden 22 in (56 cm) and Southend
21 in (53 cm). Clacton has an average summer temperature of
69°F (20°C) whilst its average minimum winter temperature is
32.3°F (0°C). Hie average maximum winter temperature is
42.3°F (5-6 °C).
Special mention must be made of Epping Forest. Owing to
its proximity to London it has received more attention bryolo-
gically than most parts of Essex. To-day it covers approximately